Shifting air-traffic control from the Federal Aviation Administration to a private corporation would raise the deficit $46 billion over the next decade, under President Trump’s budget proposal released Tuesday.

The budget acknowledged the $46 billion because of projected growth at the agency, but argued that the actual spending difference would be smaller, at about $20 billion over 10 years, based on historical trends. Wherever the figures end up, the budget said changing the governance and structure of air traffic control is key to accommodate projected growth in air traffic.

“We take this very seriously,” Mick Mulvaney, director of the Office of Management and Budget, said Monday. “This is a message from the president of the United States to the Congress that says, ‘Look, here are my priorities in terms of where I want to spend more. Here is where we shouldn’t be spending nearly as much. And here are some of the big ticket items.’”

Congress will spend months debating the proposals and may decide to change or ignore them. But the budget sets out the administration’s priorities before lawmakers decide where to actually spend money.

Democratic lawmakers have criticized the proposal to privatize air-traffic control. Lawmakers called appropriators who make spending decisions have voiced opposition to moving air-traffic control out of FAA because of the lack of congressional oversight.

“Why on Earth would we spend billions of taxpayer dollars to fix an air traffic control system that isn’t broken and is safe?” asked Sen. Bill Nelson of Florida, the top Democrat on the Senate transportation committee that oversees the airline industry.

Rep. Peter DeFazio of Oregon, the top Democrat on the House Transportation and Infrastructure Committee, blasted the budget for cutting $135 million from FAA’s budget in the year starting Oct. 1, which he said would jeopardize critical functions and investments in research, development and equipment.

“This budget does not make good on the president’s promises when it comes to transportation,” DeFazio said.

Even without the budget proposal, Congress planned to debate whether to change air-traffic control as part of FAA legislation that expires Sept. 30.

Most major airlines urge the move to a corporation like one in Canada, where a board of industry stakeholders runs the system. The goal is to provide more stable funding than annual political fights in Congress, which led to shutdowns and furloughs in recent years, and modernize equipment more efficiently.

“In order to remain competitive, we need a 21st century infrastructure with modern 21st century technology,” Rep. Bill Shuster, R-Pa., said Wednesday at a hearing about air-traffic control at the House Transportation and Infrastructure Committee, which he heads. “But the fact is, the FAA’s infrastructure is increasingly obsolete and its technology is still cemented in the last century.”

But DeFazio cited government watchdogs at the hearing that found FAA is making strides in modernizing from ground-based radar to satellite-based navigation. DeFazio noted that airlines have had 36 computer outages since 2015.

“Now splitting this agency in half does not make sense to me,” DeFazio said.